Silica-reinforced rubber compounded with an alkoxysilane and a strong organic base

ABSTRACT

Improved tensile mechanical and dynamic viscoelastic properties of silica-reinforced sulfur vulcanized rubbers can be achieved by compounding elastomers with silica in the presence of an alkoxysilane and a catalytic amount of a strong organic base. The strong organic base acts as a catalyst to accelerate the alkoxysilane-silica reaction, especially at high compounding temperatures, resulting in rubber compounds that exhibit reduced compound viscosity, improved dispersion of silica, and reduced filler flocculation after compounding, and reduced hysteresis and improved abrasion resistance in the vulcanized product, compared to similar compounds prepared at the temperature without the strong organic base catalyst.

FIELD OF THE INVENTION

The invention generally relates to vulcanizable elastomeric compoundscontaining silica as a reinforcing filler.

BACKGROUND OF THE INVENTION

When producing elastomeric compositions for use in rubber articles, suchas tires, power belts, and the like, it is desirable that theseelastomeric compositions are easily processable during compounding andhave a high molecular weight with a controlled molecular weightdistribution, glass transition temperature (T_(g)) and vinyl content. Itis also desirable that reinforcing fillers, such as silica and/or carbonblack, be well dispersed throughout the rubber in order to improvevarious physical properties, such as the compound Mooney viscosity,elastic modulus, tangent delta (tan δ), and the like. Rubber articles,especially tires, produced from vulcanized elastomers exhibiting theseimproved properties will have reduced hysteresis, better rollingresistance, snow and ice traction, wet traction, and improved fueleconomy for vehicles equipped with such tires.

With the increasing use of silica as a reinforcing filler for rubber,filler dispersion in rubber stocks has become a major concern. Becausepolar silanol groups on the surface of silica particles tend toself-associate, reagglomeration of silica particles can occur aftercompounding, leading to poor silica dispersion and a high compoundviscosity. The strong silica filler network results in a rigid uncuredcompound that is difficult to process in extrusion and formingoperations. Previous attempts at preparing readily processable,vulcanizable silica-filled rubber stocks containing natural rubber ordiene polymer and copolymer elastomers have focused on the use, duringcompounding, of bifunctional silica coupling agents having a moiety(e.g., an alkoxysilyl group) reactive with the silica surface, and amoiety (e.g., a mercapto, amino, vinyl, epoxy or sulfide group) thatbinds to the elastomer. Well known examples of such silica couplingagents are mercaptosilanes, bis(trialkoxysilylorgano) polysulfides, suchas bis(3-triethoxysilylpropyl) tetrasulfide which is sold commerciallyas Si69 by Degussa, and 3-thiocyanatopropyl trimethoxysilane. Thesebifunctional silica coupling agents offer excellent coupling betweenrubber and silica, resulting in rubbers having improved wet ice skidresistance, rolling resistance and tread wear.

However, there are disadvantages to the use of bifunctional silicacoupling agents. For example, the high chemical reactivity of the —SHfunctions of the mercaptosilanes with organic polymers can lead tounacceptably high viscosities during processing and to premature curing(scorch). The tendency of a rubber compound to scorch makes compoundingand processing more difficult. Mixing and milling must be done morequickly, yet at lower temperatures (e.g., 120° C. to 145° C.), so thatthe compound will not begin to vulcanize before it is shaped or molded.Rubber compounds employing bis(trialkoxysilylorgano) tetrasulfide silicacoupling agents, such as Si69, must be mixed at a temperature below 165°C., if an irreversible thermal degradation of the polysulfide functionof the coupling agent and premature curing of the mixture are to beavoided. The upper processing temperature limitations of thebifunctional silica coupling agents result in a marked reduction in themechanical activity of mixing which is essential for an optimumdispersion of the silica throughout the polymer matrix. Therefore,compared with carbon black-filled compositions, tread compounds havinggood silica dispersion require a longer mixing time at a lowertemperature to achieve improved performance, resulting in decreasedproduction and increased expense. Moreover, bothbis(trialkoxysilylorgano) polysulfide and mercaptosilane silica couplingagents are expensive.

Another disadvantage of the use of bis(trialkoxysilylorgano)tetrasulfide and mercaptosilane silica coupling agents is that the upperprocessing temperature limitations result in a relatively slow rate ofthe chemical reaction between the alkoxysilyl portion of the silicacoupling agents and the silica (the alkoxysilane-silica reaction).Because this reaction results in the release of a substantial amount ofalcohol, a slow reaction rate results in the presence of unreactedalkoxysilyl groups in the compounded product that are then available tofurther react with the silica and moisture during storage, extrusion,tire build, and/or curing, resulting in an undesirable increase in thecompound viscosity, and a shorter shelf life. Moreover, the continuingreaction in the compound evolves more alcohol, resulting in porous zonesor blisters which can form surface defects in the resulting formedrubber articles and/or can impair the dimensional stability of treadsduring extrusion and tire building. As a result, a low tread stripdrawing speed must be maintained to ensure that the drawn productconforms with specifications, resulting in a further decrease inproduction and concomitant increase in costs.

To address the expense and other problems related to bifunctional silicacoupling agents, recent approaches to improving dispersion of silica inrubber compounds have been directed to reducing or replacing the use ofsuch silica coupling agents by employing silica dispersing aids, such asmonofunctional silica shielding agents (e.g., silica hydrophobatingagents that chemically react with the surface silanol groups on thesilica particles but are not reactive with the elastomer) and agentswhich physically shield the silanol groups, to prevent reagglomeration(flocculation) of the silica particles after compounding. For example,silica dispersing aids, such as alkyl alkoxysilanes, glycols (e.g.,diethylene glycol or polyethylene glycol), fatty acid esters ofhydrogenated and non-hydrogenated C₅ and C₆ sugars (e.g., sorbitanoleates, and the like), polyoxyethylene derivatives of the fatty acidesters, and fillers such as mica, talc, urea, clay, sodium sulfate, andthe like, are the subjects of EP 890603 and EP 890606. Such silicadispersing aids can be used to replace all or part of expensivebifunctional silica coupling agents, while improving the processabilityof silica-filled rubber compounds by reducing the compound viscosity,increasing the scorch time, and reducing silica reagglomeration. Toachieve a satisfactory cure of the rubber compound, the use of silicadispersing aids includes employing an increased amount of sulfur in amixing step when curing agents are added to the composition, to replacesulfur that otherwise would have been supplied by a sulfur-containingsilica coupling agent.

An advantage of the use of silica dispersing aids during compounding ofelastomers with silica is that, unlike the bifunctional silica couplingagents described above, the dispersing aids do not contain sulfur and,thus, they can be used at high temperature, e.g., about 165° C. to about200° C., in the absence of curing agents, without increasing the risk ofpremature curing. At these high temperatures, the reaction between thesilica and alkoxysilyl groups of alkyl alkoxysilane silica dispersingaids is accelerated, resulting in an increase in the amount of alcoholevolved and evaporated during compounding, and a decrease in evolutionof alcohol from the compound during storage, extrusion, curing and tirebuild.

SUMMARY OF THE INVENTION

Unexpectedly, it has been discovered that improvements in the tensilemechanical properties and dynamic viscoelastic properties ofsilica-reinforced sulfur-vulcanized rubbers can be achieved bycompounding elastomers with silica, in the presence of an alkoxysilaneand a catalytic amount of a strong organic base. A strong organic base,for purposes of this invention, is defined as an organic base preferablyhaving a pK_(a) of greater than about 10 and, preferably, about 11 toabout 18. In optimal systems, the strong organic base has a pK_(a)greater than about 12. The strong organic base acts as a catalyst toaccelerate the alkoxysilane-silica reaction, and this reaction proceedsmore rapidly, the higher the compounding temperature. For example, whenmercaptosilane silica coupling agents, such asγ-mercaptoalkyltrialkoxysilanes, are employed with alkyl alkoxysilanesilica dispersing aids, in the amounts and ratios described below,compounding temperatures can range from about 130° C. to about 200° C.,and preferred high compounding temperatures can range from about 155° C.to about 200° C., more preferably about 170° C. to about 200° C., andespecially about 170° C. to about 185° C.

In one embodiment, the invention provides a sulfur-vulcanizableelastomeric compound, comprising an elastomer, a reinforcing fillercomprising silica or a mixture thereof with carbon black, an alkylalkoxysilane, a mercaptosilane silica coupling agent, wherein the weightratio of the mercaptosilane to the alkyl alkoxysilane is a maximum of0.14:1, a catalytic amount of a strong organic base, and a cure agentcomprising an effective amount of sulfur to achieve a satisfactory cure.It has been discovered that a very small amount of the mercaptosilanefacilitates binding of silica by the polymer without resulting inpremature curing, the alkyl alkoxysilane provides a desirable viscosityfor processability, and the strong organic base catalyzes thealkoxysilane-silica reaction binding silica to both silanes, in theabsence of cure agents and added sulfur. In another embodiment of theinvention, the elastomer is functionalized with an alkoxysilane terminalgroup, and the strong organic base also catalyzes the binding of thesilica to the polymer via the terminal group.

In a further embodiment, the invention provides a sulfur-vulcanizableelastomeric compound, comprising an elastomer optionally having analkoxysilane terminal group, a reinforcing filler comprising silica or amixture of silica and carbon black, a silica coupling agent selectedfrom the group consisting of about 0.1% to about 20% by weight of abis(trialkoxysilyl-organo) disulfide silica coupling agent, based on theweight of the silica, and about 0.01% to about 1% by weight of abis(trialkoxysilylorgano) tetrasulfide silica coupling agent, based onthe weight of the silica; a silica dispersing aid; a catalytic amount ofthe strong organic base; and a cure agent comprising an effective amountof sulfur to achieve a satisfactory cure. The strong organic basecatalyzes the binding of silica to the silica coupling agent, and/or tothe optional silica dispersing aid if the dispersing aid has analkoxysilane group, and/or to the optional alkoxysilane terminal groupon the elastomer, in the absence of cure agents. When polysulfide silicacoupling agents are employed, compounding temperatures can range from165° C. to about 200° C., and preferred high compounding temperaturescan range from about 170° C. to about 200° C., and especially about 170°C. to about 185° C.

The sulfur-vulcanizable rubber compounds of the invention have longerscorch times, faster curing rates, and a decrease in evolution ofethanol during storage, extrusion, curing and tire build, resulting inless compound porosity with fewer blisters, and a more stable compoundviscosity during storage, than similar compounds prepared at the sametemperature in the absence of the strong organic base. Rubber compoundsproduced according to the invention method exhibit improved dynamicviscoelastic properties, especially a lower storage modulus (G′) at −20°C., a higher tan δ at 0° C., and a lower tan δ at 50° C. Such propertieshave been commonly used in the tire industry to predict tire performancein the categories of snow and ice traction (G′ at −20° C.), wet traction(tan δ at 0° C.), and rolling resistance (tan δ at 50° C.).

The invention further provides pneumatic tires comprising at least onecomponent produced from the vulcanized rubber compounds and methods forpreparing the rubber compounds.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the results of a temperature sweep test comparing thetan δ of a cured invention rubber stock (Stock 4) containing an alkylalkoxysilane, a mercaptosilane, and a strong organic base catalyst (DPG)mixed with the elastomer and silica at 175° C. in the master batch, acomparison similar stock (C-E) mixed at the temperature in the absenceof DPG in the master batch, and a comparison stock (C-D) containing aconventional amount of Si69 silica coupling agent mixed at 155° C. whereDPG is added as a secondary accelerator in the final stage of mixing.

DETAILED DESCRIPTION OF THE INVENTION

The terms elastomer, polymer and rubber are used interchangeably herein,as is customary in the rubber industry. Strong organic bases suitablefor use as a catalyst in the invention preferably have a pK_(a) ofgreater than about 10, more preferably about 11 to about 18 and,optimally, greater than about 12. The strong base can be present in thecompound in an amount of about 0.01% to about 10%, typically about 0.1%to about 5%, based on the weight of the silica. For example, thecatalytic amount of the strong organic base is typically about 0.003 perhundred parts rubber (phr) to about 8 phr, typically about 0.03 phr toabout 4 phr. Exemplary strong organic bases for use in the inventioncompounds include, but are not limited to, strong alkali metalalkoxides, such as sodium or potassium alkoxide; guanidines, such astriphenylguanidine (TPG), diphenylguanidine (DPG), di-o-tolylguanidine(DTG), N,N,N′,N′-tetramethylguanidine (TMG), and the like; and hinderedamine bases, such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),1,5-diazabicyclo[4.3.0]non-5-ene (DBN), and the like, tertiary aminecatalysts, such as N,N-dimethylcyclohexylamine, triethylenediamine,triethylamine, and the like, quaternary ammonium bases, such astetrabutylammonoium hydroxide, bisaminoethers, such asbis(dimethylaminoethyl)ethers, and the like.

It is known that guanidines, such as diphenylguanidine (DPG), can beused as a secondary accelerator with a primary accelerator (e.g., asulfenamide, a thiazol, and the like) and sulfur in the curing stage ofrubber. Without being bound by theory, it is believed that in the curingstage, the strongly basic guanidines bind to acid sites remaining on thesilica surfaces to prevent binding of zinc, primary accelerators, andother curatives to the silica, in order to allow the sulfur-polymercrosslinking reaction to occur. Thus, guanidines used as secondaryaccelerators are added, with other curatives and sulfur, in the finalstage of mixing at a temperature below the vulcanization temperature,typically not exceeding 120° C., after the major portion of thealkoxysilane-silica reaction has already occurred. In contrast, in thepresent invention, strong organic bases, such as guanidines, act ascatalysts for the alkoxysilane-silica reaction in the first, hightemperature mixing stage, in the absence of cure agents and addedsulfur.

The alkoxysilane-silica reaction catalyzed by the strong organic baserequires the presence of a silane and silica in the sulfur-vulcanizableelastomeric composition, and is accelerated at high compoundingtemperatures, such as about 155° C. to about 200° C., especially about170° C. to about 185° C., and the like. The silane can be present as analkoxysilane terminal functional group on the polymer, and/or as analkoxysilane silica dispersing aid, and/or as a sulfur-containing silicacoupling agent, such as a bis(trialkoxysilylorgano) polysulfide or amercapto-organoalkoxysilane, and the like. In one embodiment of theinvention, the elastomer has an alkoxysilane terminal group, and thestrong organic base catalyzes the reaction between the silica filler andthe alkoxysilane terminal group to increase formation of polymer-fillerbonds.

The reaction that binds silica to polymers having one to three (n)alkoxysilane terminal groups is well known and is schematicallyillustrated below.Polymer-Si—(OR)_(n)+nH₂O →Polymer-Si—(OH)_(n)+nROH   (1)Polymer-Si—(OH)_(n)+nSiO₂→Polymer-Si—(OSi)_(n)+nH₂O   (2)

In another embodiment of the invention, the —SH groups of themercaptoorganoalkoxysilane silica coupling agent associate with thepolymer during the compounding process and the strong organic basecatalyzes the reaction of the silica and the alkoxysilane portion of themercaptosilane to bind the silica to the polymer. In a preferredembodiment, the elastomer has an alkoxysilane terminal group and both ofthe foregoing alkoxysilane-silica reactions take place.

The alkoxysilane-silica reaction also occurs in the binding of silica byalkoxysilane silica dispersing aids, and by bis(trialkoxysilylorgano)disulfide silica coupling agents that can be used at high compoundingtemperatures of about 165° C. to about 200° C. without resulting inpremature scorch. The reaction also occurs in the binding of silica bybis(trialkoxysilylorgano) tetrasulfide silica coupling agents, such asSi69, and/or bis(trialkoxysilylorgano)disulfide silica coupling agentsand/or mercaptosilane silica coupling agents, and/or alkoxysilane silicadispersing aids at a temperature of 160° C. or less, although thereaction is slower at the lower temperature. It is known that the use ofa conventional coupling amount of Si69 (e.g., about 5% to about 20% byweight, based on the weight of the silica) at a temperature of 165° C.or greater results in irreversible thermal degradation of thepolysulfide function of the coupling agent and premature curing of themixture. However, bis(trialkoxysilylorgano) disulfide silica couplingagents, which are structurally similar to the tetrasulfide couplingagents but contain a preponderance of disulfide chains, have betterthermal stability because the reaction between the disulfide chain andthe polymer only occurs in the presence of added sulfur. (KGK KautschukGummi Kunststoffe 53(1), 10-23, February. 2000; Tire TechnologyInternational, pp. 52-59, March 2000). Exemplary of this category ofcoupling agents are bis(3-triethoxysilylpropyl) disulfide (“TESPD”),containing greater than 80% disulfides, and “VP Si75”, containing about75% disulfides, both available from Degussa, and Silquest® A1589,containing about 75% disulfides, available from Crompton (formerlyWitco).

Moreover, it has been discovered that very small amounts (e.g., about0.01% to about 1% by weight, based on the weight of the silica) ofbis(trialkoxysilylorgano) tetrasulfide silica coupling agents (e.g.,Si69) can be mixed with the elastomer and the silica, in the absence ofadded sulfur and cure agents, at 165° C. to about 200° C., withoutpremature curing of the compound.

In the preferred embodiment, regardless of the source of the silane, thestrong organic base catalyzes the alkoxysilane-silica reaction at highcompounding temperature to produce rubber compounds having improvedtensile mechanical and dynamic viscoelastic properties, compared tosimilar compounds prepared at the temperature in the absence of thestrong organic base.

In one embodiment of the invention, a sulfur-vulcanizable elastomericcompound of the invention comprises an elastomer optionally having analkoxysilane terminal group, a reinforcing filler comprising silica or amixture thereof with carbon black, an alkyl alkoxysilane and amercaptosilane silica coupling agent, wherein the weight ratio of themercaptosilane to the alkyl alkoxysilane is a maximum of 0.14:1, acatalytic amount of the strong organic base, and a cure agent comprisingan effective amount of sulfur to achieve a satisfactory cure.

To obtain desirable processability, tensile mechanical properties anddynamic viscoelastic properties in the rubber compounds, the weightratio of the mercaptosilane to the alkyl alkoxysilane is a maximum of0.14:1, preferably about 0.001:1 to about 0.10:1, typically about 0.01:1to about 0.10:1. These ratios provide a compound having goodprocessability as a result of the alkyl alkoxysilane silica dispersingaid, and also a satisfactory tensile modulus at 300% strain and boundrubber content, as a result of binding of the silica filler to thepolymer by the mercaptosilane. The mercaptosilane is present in thecompound in an amount of about 0.0001% to about 3% by weight, typicallyabout 0.001% to about 1.5% by weight, and especially about 0.01% toabout 1% by weight, based on the weight of the silica. It has beendiscovered that the use of such a small amount of the mercaptosilane,even at a high mixing temperature, unexpectedly does not result inpremature curing. Therefore, the mercaptosilane and alkyl alkoxysilanecan be mixed with the elastomer and silica reinforcing filler in thefirst stage of the mixing process, especially at a higher temperature(e.g., about 130° C. to about 200° C., preferably about 155° C. to about200° C., more preferably about 170° C. to about 200° C., especiallyabout 170° C. to about 185° C.) than previously allowable forconventional amounts (e.g., about 3% by weight based on the weight ofthe silica) of mercaptosilane coupling agents, resulting in a shortermixing time with a concomitant savings in production time and expense,and improved performance of the ultimate rubber product. The presence ofthe strong organic base catalyst for the alkoxysilane-silica reaction,in addition to the mercaptosilane and alkyl alkoxysilane in the desiredweight ratio range, results in an improvement in the tensile modulus at300% strain and other tensile mechanical and dynamic viscoelasticproperties of the compound and a desirable compound viscosity andprocessability, because of the acceleration of the alkoxysilane-silicareaction catalyzed by the strong organic base.

The amount of the mercaptosilane and the alkyl alkoxysilane in thecompound is based on the weight of silica present, as is known to thoseskilled in the art of rubber compounding. The alkyl alkoxysilane can bepresent in an amount of about 0.1% to about 20% by weight, based on theweight of the silica. Preferably, the alkyl alkoxysilane is present inan amount of about 0.5% to about 15% by weight and, more preferably, inan amount of about 1% to about 10% by weight, based on the weight of thesilica.

Mercaptosilanes suitable for use in this embodiment of the inventioncompounds have the formula

where X is a halogen or an alkoxy; R is C₁ to C₄ alkylene; R′ isindependently C₁ to about C₁₀ alkyl, about C₇ to about C₃₀ alkaryl,about C₅ to about C₃₀ cycloaliphatic, or C₆ to about C₂₀ aromatic; and“n” is an integer from 1 to 3. The halogen can be selected from thegroup consisting of chlorine, bromine, iodine, and fluorine, preferablychlorine. R is preferably C₁ to C₃ alkylene, X is preferably an alkoxy;and n is preferably 3.

Exemplary mercaptosilanes include, but are not limited to,1-mercaptomethyltriethoxysilane, 2-mercaptoethyltriethoxysilane,3-mercaptopropyltriethoxysilane, 3-mercaptopropylmethyldiethoxysilane,2-mercaptoethyltriproxysilane, 18-mercaptooctadecyldiethoxychlorosilane,and the like, and mixtures of any of the foregoing.

Alkyl alkoxysilanes suitable for use in the invention compounds have theformulaR¹ _(p)Si (OR²)_(4-p)where the alkoxy groups are the same or different from each other, eachR¹ independently comprises C₁ to about C₂₀ aliphatic, about C₅ to aboutC₂₀ cycloaliphatic, or about C₆ to about C₂₀ aromatic, each R²independently comprises C₁ to about C₆, and p is an integer from 1 to 3.Preferably, at least one R¹ contains from 6 to 20 carbon atoms and theremainder of the R¹ groups, if any, contain from 1 to 3 carbon atoms.Preferably, R² contains 1 to 4, more preferably 1 or 2, carbon atoms.Preferably R² is an alkyl group. More preferably, at least one R¹ ismuch larger in terms of carbon atoms than an R² contained in the alkoxygroups of the silane.

Exemplary alkyl alkoxysilanes include, but are not limited to, octyltriethoxysilane, octyl trimethoxysilane, trimethyl ethoxysilane,cyclohexyl triethoxysilane, isobutyl triethoxysilane, ethyltrimethoxysilane, cyclohexyl tributoxysilane, dimethyl diethoxysilane,methyl triethoxysilane, propyl triethoxysilane, hexyl triethoxysilane,heptyl triethoxysilane, nonyl triethoxysilane, octadecyltriethoxysilane, methyloctyl diethoxysilane, dimethyl dimethoxysilane,methyl trimethoxysilane, propyl trimethoxysilane, hexyltrimethoxysilane, heptyl trimethoxysilane, nonyl trimethoxysilane,octadecyl trimethoxysilane, methyloctyl dimethoxysilane, and mixturesthereof. Preferably, the alkyl alkoxysilane is an alkyl trialkoxysilane.More preferably, the alkyl alkoxysilane is selected from at least one ofn-octyl triethoxysilane, n-hexadecyl triethoxysilane, n-octadecyltriethoxysilane, and methyl n-octyl diethoxysilane.

Although alkyl alkoxysilanes and mercaptosilanes employing methoxysilanegroups can be used, it is preferred for environmental reasons thatethoxysilanes are employed, rather than methoxysilanes, because ethylalcohol, rather than methyl alcohol, will be released when thealkoxysilane portion of the coupling agent reacts with the surface ofthe silica particle.

In another embodiment of the invention, a sulfur-vulcanizableelastomeric compound comprises an elastomer optionally having analkoxysilane terminal group; a reinforcing filler comprising silica or amixture thereof with carbon black; a silica coupling agent selected fromthe group consisting of about 0.01% to about 1% by weight of abis(trialkoxysilylorgano)tetrasulfide silica coupling agent, based onthe weight of the silica, about 0.1% to about 20% by weight of abis(trialkoxysilylorgano)disulfide silica coupling agent, based on theweight of the silica, and mixtures thereof; a silica dispersing aid; acatalytic amount of the strong organic base; and a cure agent comprisingan effective amount of sulfur to achieve a satisfactory cure. Thecompound is preferably formed by mixing the elastomer, the silica, thesilica coupling agent, the silica dispersing aid and the strong organicbase, in the absence of the cure agent, at a temperature of 165° C. toabout 200° C. More preferably, the mixing temperature is about 170° C.to about 200° C., especially about 170° C. to about 185° C. The strongorganic base and the catalytic amounts employed in the vulcanizableelastomeric composition are the same as those described above.

Exemplary bis(trialkoxysilylorgano)disulfide silica coupling agentssuitable for use in the invention include, but are not limited to,3,3′-bis(triethoxy-silylpropyl) disulfide,3,3′-bis(trimethoxysilylpropyl) disulfide,3,3′-bis(tributoxysilyl-propyl) disulfide,3,3′-bis(tri-t-butoxysilylpropyl) disulfide,3,3′-bis(trihexoxysilyl-propyl) disulfide,2,2′-bis(dimethylmethoxysilylethyl) disulfide,3,3′-bis(diphenyl-cyclohexoxysilylpropyl) disulfide,3,3′-bis(ethyl-di-sec-butoxysilylpropyl) disulfide,3,3′-bis(propyldiethoxysilylpropyl) disulfide,12,12′-bis(triisopropoxysilylpropyl) disulfide,3,3′-bis(dimethoxyphenylsilyl-2′-methylpropyl) disulfide, and the like,and mixtures of any of the foregoing.

Exemplary bis(trialkoxysilylorgano)tetrasulfide silica coupling agentssuitable for use in the invention include, but are not limited to,bis(3-triethoxysilylpropyl) tetrasulfide, bis(2-triethoxysilylethyl)tetrasulfide, bis(3-trimethoxysilylpropyl) tetrasulfide,3-trimethoxysilylpropyl-N,N-dimethylthio-carbamoyl tetrasulfide,3-triethoxysilylpropyl-N,N-dimethylthiocarbamoyl tetrasulfide,2-triethoxysilyl-N,N-dimethylthiocarbamoyl tetrasulfide,3-trimethoxy-silylpropylbenzothiazole tetrasulfide,3-triethoxysilylpropylbenzothiazole tetrasulfide, and the like, andmixtures of any of the foregoing. Particularly preferred isbis(3-triethoxysilylpropyl)tetrasulfide.

Similarly to the alkyl alkoxysilanes and mercaptosilanes describedabove, the bis(trialkoxysilylorgano) tetrasulfide and disulfide silicacoupling agents employing methoxysilane groups can be used; however, itis preferred for environmental reasons that ethoxysilanes are employed,rather than methoxysilanes, because ethyl alcohol, rather than methylalcohol, will be released when the alkoxysilane portion of the couplingagent reacts with the surface of the silica particle.

The bis(trialkoxysilylorgano) disulfide silica coupling agent can bepresent in the compound in an amount of about 0.1% to about 20% byweight, typically about 0.5% to about 15% by weight, especially about 1%to about 10% by weight, based on the weight of the silica. Thebis(trialkoxysilylorgano)tetrasulfide silica coupling agent can bepresent in the compound in an amount of about 0.01% to about 1% byweight, typically about 0.05% to about 1% by weight, especially about0.1% to about 1% by weight, based on the weight of the silica.

As discussed below, in all of the embodiments of the invention, thepolymer preferably is an elastomer selected from the group consisting ofhomopolymers of conjugated diene monomers, and copolymers andterpolymers of the conjugated diene monomers with monovinyl aromaticmonomers and trienes. Exemplary elastomers include, but are not limitedto, polyisoprene, polystyrene, polybutadiene, butadiene-isoprenecopolymer, butadiene-isoprene-styrene terpolymer, isoprene-styrenecopolymer, and styrene-butadiene copolymer.

Exemplary silica dispersing aids suitable for use in the inventioninclude, but are not limited to an alkyl alkoxysilane, a fatty acidester of a hydrogenated or non-hydrogenated C₅ or C₆ sugar, apolyoxyethylene derivative of a fatty acid ester of a hydrogenated ornon-hydrogenated C₅ or C₆ sugar, and mixtures thereof or a mineral ornon-mineral additional filler, as described in greater detail below. Thealkyl alkoxysilane is the same as described above, and is preferably analkyl triethoxysilane. It will be appreciated by those skilled in theart, that the total amount of the alkyl alkoxysilane employed in theembodiment of the invention where the silica coupling agent is amercaptosilane will not exceed the maximum allowed to produce a maximummercaptosilane/alkyl alkoxysilane ratio of 0.14:1. In the embodiment ofthe invention employing the bis(trialkoxysilylorgano) disulfide and/ortetrasulfide silica coupling agent, the alkyl alkoxysilane can bepresent in the compound in an amount of about 0.1% to about 25% byweight, especially about 0.1% to about 15% by weight, based on theweight of the silica.

Exemplary fatty acid esters of hydrogenated and non-hydrogenated C₅ andC₆ sugars (e.g., sorbose, mannose, and arabinose) that are useful assilica dispersing aids include, but are not limited to, the sorbitanoleates, such as sorbitan monooleate, dioleate, trioleate andsesquioleate, as well as sorbitan esters of laurate, palmitate andstearate fatty acids. Fatty acid esters of hydrogenated andnon-hydrogenated C₅ and C₆ sugars are commercially available from ICISpecialty Chemicals (Wilmington, Del.) under the trade name SPAN®.Representative products include SPAN® 60 (sorbitan stearate), SPAN® 80(sorbitan oleate), and SPAN® 85 (sorbitan trioleate). Other commerciallyavailable fatty acid esters of sorbitan are also available, such as thesorbitan monooleates known as Alkamul® SMO; Capmul® O; Glycomul® O;Arlacel® 80; Emsorb® 2500; and S-Maz® 80. A useful amount of theseadditional silica dispersing aids when used with thebis(trialkoxysilylorgano) polysulfide silica coupling agents is about0.1% to about 25% by weight based on the weight of the silica, withabout 0.5% to about 20% by weight being preferred, and about 1% to about15% by weight based on the weight of the silica being more preferred. Inthe alkyl alkoxysilane and mercaptosilane embodiment of the invention,it may be desirable to use about 0.1% to about 20% by weight of thefatty acid ester based on the weight of the silica. Esters of polyols,including glycols such as polyhydroxy compounds and the like, in thesame quantities, are also useful in all invention embodiments.

Exemplary polyoxyethylene derivatives of fatty acid esters ofhydrogenated and non-hydrogenated C₅ and C₆ sugars include, but are notlimited to, polysorbates and polyoxyethylene sorbitan esters, which areanalogous to the fatty acid esters of hydrogenated and non-hydrogenatedsugars noted above except that ethylene oxide groups are placed on eachof the hydroxyl groups. Representative examples of polyoxyethylenederivatives of sorbitan include POE® (20) sorbitan monooleate,Polysorbate® 80, Tween® 80, Emsorb® 6900, Liposorb® O-20, T-Maz® 80, andthe like. The Tween® products are commercially available from ICISpecialty Chemicals. Generally, a useful amount of these optional silicadispersing aids is about 0.1% to about 25% by weight based on the weightof the silica, with about 0.5% to about 20% by weight being preferred,and about 1% to about 15% by weight based on the weight of the silicabeing more preferred. In the alkyl alkoxysilane and mercaptosilaneembodiment of the invention, it may be desirable to use about 0.1% toabout 20% by weight of the polyoxyethylene derivative based on theweight of the silica.

The silica coupling agents, the alkyl alkoxysilanes, the fatty acidesters and their polyoxyethylene derivatives, and the strong organicbase catalysts, can be fully or partially supported by the reinforcingfiller. The ratio of the dispersing aid or catalyst to the reinforcingfiller is not critical. If the dispersing aid is a liquid, a suitableratio of dispersing aid to filler is that which results in a suitablydry material for addition to the elastomer. For example, the ratio canbe about 1/99 to about 70/30, about 20/80 about 60/40, about 50/50, andthe like.

Certain additional fillers can be utilized according to the presentinvention as processing aids, including mineral fillers, such as clay(hydrous aluminum silicate), talc (hydrous magnesium silicate), aluminumhydrate [Al(OH)₃] and mica, as well as non-mineral fillers such as ureaand sodium sulfate. Preferred micas principally contain alumina andsilica, although other known variants are also useful. The foregoingadditional fillers are optional and can be utilized in the amount ofabout 0.5 to about 40 phr, preferably in an amount of about one to about20 phr and, more preferably in an amount of about one to about 10 phr.These additional fillers can also be used as non-reinforcing fillers tosupport the strong organic base catalysts, as well as any of the silicadispersing aids, and silica coupling agents described above. As with thesupport of the silica dispersing aid on the reinforcing filler, asdescribed above, the ratio of dispersing aid to non-reinforcing filleris not critical. For example, the ratio can be about 1/99 to about70/30, about 20/80 about 60/40, about 50/50, and the like.

The elastomeric compositions of the invention are preferably compoundedwith reinforcing fillers, such as silica, or a mixture of silica andcarbon black. Examples of suitable silica reinforcing filler include,but are not limited to, precipitated amorphous silica, wet silica(hydrated silicic acid), dry silica (anhydrous silicic acid), fumedsilica, calcium silicate, and the like. Other suitable fillers includealuminum silicate, magnesium silicate, and the like. Among these,precipitated amorphous wet-process, hydrated silicas are preferred.These silicas are so-called because they are produced by a chemicalreaction in water, from which they are precipitated as ultrafine,spherical particles. These primary particles strongly associate intoaggregates, which in turn combine less strongly into agglomerates. Thesurface area, as measured by the BET method gives the best measure ofthe reinforcing character of different silicas. For silicas of interestfor the present invention, the surface area should be about 32 m²/g toabout 400 m²/g, with the range of about 100 m²/g to about 250 m²/g beingpreferred, and the range of about 150 m²/g to about 220 m²/g being mostpreferred. The pH of the silica filler is generally about 5.5 to about 7or slightly over, preferably about 5.5 to about 6.8.

Silica can be employed in the amount of about one to about 100 parts byweight per hundred parts of the elastomer (phr), preferably in an amountof about five to about 80 phr and, more preferably, in an amount ofabout 30 to about 80 phr. The useful upper range is limited by the highviscosity imparted by fillers of this type. Some of the commerciallyavailable silicas which can be used include, but are not limited to,Hi-Sil® 190, Hi-Sil® 210, Hi-Sil® 215, Hi-Sil® 233, Hi-Sil® 243, and thelike, produced by PPG Industries (Pittsburgh, Pa.). A number of usefulcommercial grades of different silicas are also available from DegussaCorporation (e.g., VN2, VN3), Rhone Poulenc (e.g., Zeosil® 1165MP), andJ. M. Huber Corporation.

The elastomers can be compounded with all forms of carbon black in amixture with the silica. The carbon black can be present in amountsranging from about one to about 50 phr, with about five to about 35 phrbeing preferred. The carbon blacks can include any of the commonlyavailable, commercially-produced carbon blacks, but those having asurface area (EMSA) of at least 20 m²/g and, more preferably, at least35 m²/g up to 200 m²/g or higher are preferred. Surface area values usedin this application are determined by ASTM D-1765 using thecetyltrimethyl-ammonium bromide (CTAB) technique. Among the usefulcarbon blacks are furnace black, channel blacks and lamp blacks. Morespecifically, examples of useful carbon blacks include super abrasionfurnace (SAF) blacks, high abrasion furnace (HAF) blacks, fast extrusionfurnace (FEF) blacks, fine furnace (FF) blacks, intermediate superabrasion furnace (ISAF) blacks, semi-reinforcing furnace (SRF) blacks,medium processing channel blacks, hard processing channel blacks andconducting channel blacks. Other carbon blacks which can be utilizedinclude acetylene blacks. A mixture of two or more of the above blackscan be used in preparing the carbon black products of the invention.Typical suitable carbon blacks are N-110, N-220, N-339, N-330, N-351,N-550, N-660, as designated by ASTM D-1765-82a. The carbon blacksutilized in the preparation of the vulcanizable elastomeric compositionsof the invention can be in pelletized form or an unpelletized flocculentmass. Preferably, for more uniform mixing, unpelletized carbon black ispreferred.

In one embodiment of the invention, the sulfur-vulcanized elastomericcompound of the invention is prepared by the steps of (a) mixingtogether at a temperature of about 130° C. to about 200° C. in theabsence of added sulfur and cure agents, an elastomer optionally havingan alkoxysilane terminal group, a reinforcing filler comprising silicaor a mixture thereof with carbon black, an alkyl alkoxysilane, and amercaptosilane, wherein the ratio of the mercaptosilane to the alkylalkoxysilane is a maximum of 0.14:1, and a catalytic amount of a strongorganic base; (b) allowing the mixture to cool below the mixingtemperature; (c) mixing the mixture obtained in step (b), at atemperature lower than a vulcanization temperature, with a cure agentand an effective amount of sulfur to achieve a satisfactory cure; and(d) curing the mixture obtained in step (c). The compound is usuallycured at about 140° C. to about 190° C. for about 5 to about 120minutes.

In another embodiment of the invention, a method for preparing asulfur-vulcanized elastomeric compound, comprises the steps of: (a)mixing together at a temperature of 165° C. to about 200° C. in theabsence of added sulfur and cure agents, an elastomer optionally havingan alkoxysilane terminal group, a reinforcing filler comprising silicaor a mixture thereof with carbon black, a silica coupling agent selectedfrom the group consisting of about 0.01% to about 1% by weight of abis(trialkoxysilylorgano)tetrasulfide silica coupling agent, based onthe weight of the silica, about 0.1% to about 20% by weight of abis(trialkoxysilylorgano) disulfide silica coupling agent, based on theweight of the silica, and mixtures thereof, a silica dispersing aid, anda catalytic amount of a strong organic base; (b) allowing the mixture tocool below the mixing temperature; (c) mixing the mixture obtained instep (b), at a temperature lower than a vulcanization temperature, witha cure agent and an effective amount of sulfur to achieve a satisfactorycure; and (d) curing the mixture obtained in step (c).

In particular, the initial step in each method embodiment requires thatthe mixture reaches a temperature from 165° C. to about 200° C.,preferably about 170° C. to about 200° C. and, more preferably, about170° C. to about 185° C. Alternatively, the initial mixing step caninclude at least two substeps. That is, in the alkyl alkoxysilane andmercaptosilane embodiment, the initial mixing step can comprise thesubsteps of (i) mixing together at a temperature of about 130° C. toabout 200° C., the elastomer, at least a portion of the silica, at leasta portion of the alkyl alkoxysilane and at least a portion of themercaptosilane, and the strong organic base, (ii) cooling the mixturebelow the mixing temperature; and (iii) mixing the mixture obtained instep (ii) with the remainder of the silica, if any, and the remainder ofthe alkyl alkoxysilane and mercaptosilane, if any, at a temperature of130° C. to about 200° C. Similarly, in the bis(trialkoxysilylorgano)polysulfide embodiment, the initial mixing step can comprises thesubsteps of (i) mixing together at temperature of 165° C. to about 200°C., the elastomer, at least a portion of the silica, at least a portionof the bis(trialkoxysilylorgano) disulfide or bis(trialkoxysilylorgano)tetrasulfide silica coupling agent, at least a portion of the silicadispersing aid, and the strong organic base; (ii) cooling the mixturebelow the mixing temperature; and (iii) mixing the mixture obtained instep (ii) with the remainder of the silica, if any, and the remainder ofthe silica coupling agent and/or the remainder of the silica dispersingaid, if any, at 165° C. to about 200° C. The temperatures achieved bythe at least two substeps can be the same or different from each other,within the temperature range. As disclosed above, the preferredtemperature range is about 170° C. to about 200° C., especially about170° C. to about 185° C.

Each of the methods can further include a remill step in which either noingredients are added to the first mixture, or non-curing ingredientsare added, in order to reduce the compound viscosity and improve thedispersion of the silica reinforcing filler. The temperature of theremill step is typically about 130° C. to about 175° C., especiallyabout 145° to about 165° C.

The final step of the mixing process is the addition of cure agents tothe mixture, including an effective amount of sulfur to achieve asatisfactory cure of the final compound. The temperature at which thefinal mixture is mixed must be below the vulcanization temperature inorder to avoid unwanted precure of the compound. Therefore, thetemperature of the final mixing step should not exceed about 120° C. andis typically about 40° C. to about 120° C., preferably about 60° C. toabout 110° C. and, especially, about 75° C. to about 100° C.

The order of addition of the silica, alkyl alkoxysilane, themercaptosilane, and the strong organic base catalyst to the elastomer inthe mixer in the initial step of the method is not critical. The alkylalkoxysilane and/or the mercaptosilane and/or the strong organic basecan be added prior to or after the addition of the silica to theelastomer. In one embodiment, a portion of the silica and themercaptosilane and/or the alkyl alkoxysilane are added simultaneously tothe mixer. For example, the mercaptosilane and/or the alkyl alkoxysilanecan be partially or fully supported on the silica and/or the carbonblack reinforcing filler. An exemplary commercial product containing amercaptosilane supported on silica is available from PPG Industries, asCiptane® 255LD. The ratio of the amount of supported alkyl alkoxysilaneto the filler is not critical. If the allyl alkoxysilane is a liquid, asuitable ratio of supported silane to filler is that which results in asuitably dry material for addition to the elastomer. For example, theratio can be about 1/99 to about 70/30, about 20/80, about 60/40, about50/50, and the like.

Similarly, the order of addition of the silica,bis(trialkoxysilylorgano) tetrasulfide silica coupling agent orbis(trialkoxysilylorgano) disulfide silica coupling agent, silicadispersing aid, and the strong organic base to the mixer in the initialstep of the method is not critical. The silica coupling agent and/or thestrong organic base and/or the silica dispersing aid can be added priorto or after the addition of the silica to the elastomer. The silicacoupling agent and/or the silica dispersing aid and/or the strongorganic base can be partially or fully supported on the silica and/orthe carbon black reinforcing filler. An exemplary commercial productcontaining Si69 supported as a 50/50 blend on carbon black is availablefrom Degussa, as X50S. The ratio of the amount of supported silicacoupling agent to the filler is not critical.

The use of an alkyl alkoxysilane with a very small amount ofmercaptosilane requires an appropriate adjustment in the amount ofsulfur added to the elastomeric compound to achieve a satisfactory cureof the compound. In particular, the amount of the mercaptosilaneemployed in the present invention provides substantially less sulfurthan required for a satisfactory cure. Moreover, in the embodiment ofthe invention wherein a less than conventional amount of thebis(trialkoxysilylorgano) tetrasulfide silica coupling agent isemployed, an adjustment in the amount of sulfur added in the final stepof the mixing process is required. An effective amount of sulfur in anyof the invention compositions would provide a property of the curedcompound that is approximately equal to the same property of asatisfactorily cured compound containing a conventional amount of Si69(i.e., about 5% to about 20% by weight, based on the weight of thesilica). Cured properties for comparison include, but are not limitedto, the value of the 300% modulus (psi), the molecular weight betweencrosslinks (M_(c), g/mol), and the like, and other cured compoundproperties that are well known to those skilled in the art of rubbermaking. The increased amount of sulfur to compensate for the reducedavailability of sulfur from the mercaptosilane orbis(trialkoxy-silylorgano) tetrasulfide silica coupling agents will varyfrom composition to composition, depending on the amount of silica andthe amount of silica coupling agent present in the formulation. Based onthe disclosure contained herein, and in the examples of inventioncompositions described below, one skilled in the art of rubbercompounding can easily determine the effective amount of sulfur requiredfor a satisfactory cure of the compound without undue experimentation.The additional sulfur can take any form, including soluble sulfur,insoluble sulfur, or any of the sulfur-donating compounds described asvulcanizing agents below, or mixtures of the foregoing.

The compounds produced by the each of the foregoing methods preferablyexhibit an improved tensile mechanical or dynamic viscoelastic propertycompared to a similar compound mixed at the same temperature in theabsence of the strong organic base, the property selected from the groupof properties consisting of about a 1% to about a 10% decrease in Mooneyviscosity after compounding, about a 10% to about a 40% decrease infiller flocculation after compounding, as measured by ΔG′; reducedhysteresis as measured by about a 3% to about a 20% decrease in tan δ at65° C. and/or tan δ at 50° C.; about a 3% to about a 20% increase in thetensile modulus at 300% strain, and combinations thereof.

The tensile mechanical properties of the invention compounds also arecomparable to, or improved over, similar compounds mixed with aconventional amount of a bifunctional silica coupling agent, such asSi69, at 160° C. or less, in the absence of the strong organic base.

The present invention can be used in conjunction with any solutionpolymerizable or emulsion polymerizable elastomer. Solution and emulsionpolymerization techniques are well known to those of ordinary skill inthe art. For example, conjugated diene monomers, monovinyl aromaticmonomers, triene monomers, and the like, can be anionically polymerizedto form conjugated diene polymers, or copolymers or terpolymers ofconjugated diene monomers and monovinyl aromatic monomers (e.g.,styrene, alpha methyl styrene and the like) and triene monomers. Thus,the elastomeric products can include diene homopolymers from monomer Aand copolymers thereof with monovinyl aromatic monomers B. Exemplarydiene homopolymers are those prepared from diolefin monomers having fromabout four to about 12 carbon atoms. Exemplary vinyl aromatic copolymersare those prepared from monomers having from about eight to about 20carbon atoms. Copolymers can comprise from about 99 percent to about 50percent by weight of diene units and from about one to about 50 percentby weight of monovinyl aromatic or triene units, totaling 100 percent.The polymers, copolymers and terpolymers of the present invention canhave 1,2-microstructure-contents ranging from about 10 percent to about80 percent, with the preferred polymers, copolymers or terpolymershaving 1,2-microstructure content of from about 25 to 65 percent, basedupon the diene content. The elastomeric copolymers are preferably randomcopolymers which result from simultaneous copolymerization of themonomers A and B with randomizing agents, as is known in the art.

Preferred polymers for use in a vulcanized elastomeric compound of theinvention include polyisoprene, polystyrene, polybutadiene,butadiene-isoprene copolymer, butadiene-isoprene-styrene terpolymer,isoprene-styrene copolymer, and styrene-butadiene copolymer.

Anionic polymerization initiators for use in polymerizing theanionically polymerizable monomers include, but are not limited to,organo-sodium, organo-potassium, organo-tin-lithium, organo-lithium,dialkylimido-lithium and cycloalkylimido-lithium initiators. As anexample of such initiators, organo-lithium compounds useful in thepolymerization of 1,3-diene monomers are hydrocarbyl lithium compoundshaving the formula RLi, where R represents a hydrocarbyl groupcontaining from one to about 20 carbon atoms, and preferably from about2 to about 8 carbon atoms. Although the hydrocarbyl group is preferablyan aliphatic group, the hydrocarbyl group can also be cycloaliphatic oraromatic. The aliphatic group can be a primary, secondary, or tertiarygroup, although the primary and secondary groups are preferred. Examplesof aliphatic hydrocarbyl groups include methyl, ethyl, n-propyl,isopropyl, n-butyl, sec-butyl, t-butyl, n-amyl, sec-amyl, n-hexyl,sec-hexyl, n-heptyl, n-octyl n-nonyl, n-dodecyl, and octadecyl. Thealiphatic group can contain some unsaturation, such as allyl, 2-butenyl,and the like. Cycloalkyl groups are exemplified by cyclohexyl,methylcyclohexyl, ethylcyclohexyl, cycloheptyl, cyclopentylmethyl, andmethylcyclopentylethyl. Examples of aromatic hydrocarbyl groups includephenyl, tolyl, phenylethyl, benzyl, naphthyl, phenyl cyclohexyl, and thelike.

Specific examples of organo-lithium compounds which are useful asanionic initiators in the polymerization of the monomers listed above,especially conjugated dienes include, but are not limited to, n-butyllithium, n-propyl lithium, iso-butyl lithium, tert-butyl lithium,tributyl tin lithium (described in co-owned U.S. Pat. No. 5,268,439),amyl-lithium, cyclohexyl lithium, and the like. Other suitableorgano-lithium compounds for use as anionic initiators are well known tothose skilled in the art. A mixture of different lithium initiatorcompounds also can be employed. The preferred organo-lithium initiatorsare n-butyl lithium, tributyl tin lithium and “in situ” produced lithiumhexamethyleneimide initiator prepared by reacting hexamethyleneimine andn-butyl lithium (described in co-owned U.S. Pat. No. 5,496,940).

The amount of initiator required to effect the desired polymerizationcan be varied over a wide range depending upon a number of factors, suchas the desired polymer molecular weight, the desired 1,2- and1,4-content of the polydiene, and the desired physical properties forthe polymer produced. In general, the amount of initiator utilized canvary from as little as 0.2 millimoles (mM) of lithium per 100 grams ofmonomers up to about 100 mM of lithium per 100 grams of monomers,depending upon the desired polymer molecular weight.

Polymerization is usually conducted in a conventional solvent foranionic polymerizations, such as hexane, cyclohexane, benzene and thelike. Various techniques for polymerization, such as semi-batch andcontinuous polymerization can be employed.

In order to promote randomization in co-polymerization and to increasevinyl content, a polar coordinator can optionally be added to thepolymerization ingredients. Amounts range from zero to about 90 or moreequivalents per equivalent of lithium. The amount depends upon the typeof polar coordinator that is employed, the amount of vinyl desired, thelevel of styrene employed and the temperature of the polymerizations, aswell as the selected initiator. Compounds useful as polar coordinatorsare organic and include tetrahydrofuran, linear and cyclic oligomericoxolanyl alkanes such as 2-2′-di(tetrahydrofuryl) propane, dipiperidylethane, hexamethyl phosphoramide, N-N′-dimethyl piperazine, diazabicyclooctane, dimethyl ether, diethyl ether, tributyl amine and the like. Thelinear and cyclic oligomeric oxolanyl alkane polar coordinators aredescribed in U.S. Pat. No. 4,429,091, the subject matter of whichregarding polar coordinators is incorporated herein by reference. Othercompounds useful as polar coordinators include those having an oxygen ornitrogen hetero-atom and a non-bonded pair of electrons. Examplesinclude dialkyl ethers of mono and oligo alkylene glycols; “crown”ethers; and tertiary amines, such as tetramethylethylene diamine(TMEDA).

Polymerization is begun by charging a blend of the monomer(s) andsolvent to a suitable reaction vessel, followed by the addition of thepolar coordinator and the initiator previously described. The procedureis carried out under anhydrous, anaerobic conditions. Often, it isconducted under a dry, inert gas atmosphere. The polymerization can becarried out at any convenient temperature, such as about 0° C. to about150° C. For batch polymerizations, it is preferred to maintain the peaktemperature at from about 50° C. to about 150° C. and, more preferably,from about 60° C. to about 100° C. Polymerization is allowed to continueunder agitation for about 0.15 hours to 24 hours. After polymerizationis complete, the product is terminated by a quenching agent, anendcapping agent and/or a coupling agent, as described herein below. Theterminating agent is added to the reaction vessel, and the vessel isagitated for about 0.1 hours to about 4.0 hours. Quenching is usuallyconducted by stirring the polymer and quenching agent for about 0.01hours to about 1.0 hour at temperatures of from about 20° C. to about120° C. to ensure a complete reaction. Polymers terminated with analkoxysilane functional group, as discussed herein below, aresubsequently treated with alcohol or other quenching agent.

Lastly, the solvent is removed from the polymer by conventionaltechniques such as drum drying, extruder drying, vacuum drying or thelike, which can be combined with coagulation with water, alcohol orsteam. If coagulation with water or steam is used, oven drying can bedesirable.

One way to terminate the polymerization reaction is to employ a proticquenching agent to give a monofunctional polymer chain. Quenching can beconducted in water, steam or an alcohol such as isopropanol, or anyother suitable method. Quenching can also be conducted with a functionalterminating agent, resulting in a difunctional polymer. Any compoundsproviding terminal functionality (i.e., endcapping) that are reactivewith the polymer bound carbon-lithium moiety can be selected to providea desired functional group. Examples of such compounds are alcohols,substituted aldimines, substituted ketimines, Michler's ketone,1,3-dimethyl-2-imidazolidinone, 1-alkyl substituted pyrrolidinones,1-aryl substituted pyrrolidinones, tin tetrachloride, tributyl tinchloride, carbon dioxide, and mixtures thereof. Further examples ofreactive compounds include the terminators described in co-owned U.S.Pat. Nos. 5,521,309 and 5,066,729, the subject matter of which,pertaining to terminating agents and terminating reactions, is herebyincorporated by reference. Other useful terminating agents can includethose of the structural formula (R)_(z)ZX_(b), where Z is tin orsilicon. It is preferred that Z is tin. R is an alkyl having from about1 to about 20 carbon atoms; a cycloalkyl having from about 3 to about 20carbon atoms; an aryl having from about 6 to about 20 carbon atoms, oran aralkyl having from about 7 to about 20 carbon atoms. For example, Rcan include methyl, ethyl, n-butyl, neophyl, phenyl, cyclohexyl or thelike. X is a halogen, such as chlorine or bromine, or alkoxy (—OR), “a”is an integer from zero to 3, and “b” is an integer from one to 4, wherea+b=4. Examples of such terminating agents include tin tetrachloride,tributyl tin chloride, butyl tin trichloride, butyl silicon trichloride,as well as tetraethoxysilane, Si(OEt)₄, and methyl triphenoxysilane,MeSi(OPh)₃. The practice of the present invention is not limited solelyto these terminators, since other compounds that are reactive with thepolymer bound carbon-lithium moiety can be selected to provide a desiredfunctional group.

While terminating to provide a functional group on the terminal end ofthe polymer is preferred, it is further preferred to terminate by acoupling reaction with, for example, tin tetrachloride or other couplingagent such as silicon tetrachloride or esters. High levels of tincoupling are desirable in order to maintain good processability in thesubsequent manufacturing of rubber products. It is preferred that thepolymers for use in the vulcanizable elastomeric compositions accordingto the present invention have at least about 25 percent tin coupling.That is, about 25 percent of the polymer mass after coupling is ofhigher molecular weight than the polymer before coupling as measured,for example, by gel permeation chromatography. Preferably, beforecoupling, the polydispersity (the ratio of the weight average molecularweight to the number average molecular weight) of polymers, which can becontrolled over a wide range, is from about one to about 5, preferablyone to about 2 and, more preferably, one to about 1.5.

As noted above, various techniques known in the art for carrying outpolymerizations can be used to produce elastomers polymers suitable foruse in the vulcanizable elastomeric compositions, without departing fromthe scope of the present invention.

The preferred conjugated diene polymers, or copolymers or terpolymers ofconjugated diene monomers and monovinyl aromatic monomers, can beutilized as 100 parts of the rubber in the treadstock compound, or theycan be blended with any conventionally employed treadstock rubber whichincludes natural rubber, synthetic rubber and blends thereof Suchrubbers are well known to those skilled in the art and include syntheticpolyisoprene rubber, styrene-butadiene rubber (SBR),styrene-isoprene-butadiene rubber, styrene-isoprene rubber,butadiene-isoprene rubber, polybutadiene, butyl rubber, neoprene,ethylene-propylene rubber, ethylene-propylene-diene rubber (EPDM),acrylonitrile-butadiene rubber (NBR), silicone rubber, thefluoroelastomers, ethylene acrylic rubber, ethylene vinyl acetatecopolymer (EVA), epichlorohydrin rubbers, chlorinated polyethylenerubbers, chlorosulfonated polyethylene rubbers, hydrogenated nitrilerubber, tetrafluoroethylene-propylene rubber and the like. When thevulcanizable elastomeric composition of the present invention is blendedwith conventional rubbers, the amounts can vary widely with a lowerlimit comprising about ten percent to 20 percent by weight of the totalrubber. The minimum amount will depend primarily upon the physicalproperties desired.

Vulcanized elastomeric compounds of the invention are prepared by themethod described above. It is readily understood by those having skillin the art that the rubber compound would be compounded by methodsgenerally known in the rubber compounding art, such as mixing thevarious vulcanizable polymer(s) with various commonly used additivematerials such as, for example, curing agents, activators, retarders andaccelerators, processing additives, such as oils, resins, includingtackifying resins, plasticizers, pigments, additional fillers, fattyacid, zinc oxide, waxes, antioxidants, anti-ozonants, and peptizingagents. As known to those skilled in the art, depending on the intendeduse of the sulfur vulcanizable and sulfur vulcanized material (rubbers),the additives mentioned above are selected and commonly used inconventional amounts., in addition to other conventional rubberadditives including, for example, other fillers, plasticizers,antioxidants, cure agents and the like, using standard rubber mixingequipment and procedures.

Such elastomeric compositions, when vulcanized using conventional rubbervulcanization conditions, exhibit reduced hysteresis, which means aproduct having increased rebound, decreased rolling resistance andlessened heat build-up when subjected to mechanical stress. Productsincluding tires, power belts and the like are envisioned. Decreasedrolling resistance is, of course, a useful property for pneumatic tires,both radial as well as bias ply types and thus, the vulcanizableelastomeric compositions of the present invention can be utilized toform treadstocks for such tires. Pneumatic tires can be made accordingto the constructions disclosed in U.S. Pat. Nos. 5,866,171; 5,876,527;5,931,211; and 5,971,046, the disclosures of which are incorporatedherein by reference. The composition can also be used to form otherelastomeric tire components such as subtreads, black sidewalls, body plyskims, bead fillers and the like.

Typical amounts of tackifier resins, if used, comprise about 0.5 toabout 10 phr, usually about one to about 5 phr. Typical amounts ofcompounding aids comprise about one to about 50 phr. Such compoundingaids can include, for example, aromatic, naphthenic, and/or paraffinicprocessing oils. Typical amounts of antioxidants comprise about 0.1 toabout 5 phr. Suitable antioxidants, such as diphenyl-p-phenylenediamine,are known to those skilled in the art. Typical amounts of anti-ozonantscomprise about 0.1 to about 5 phr.

Typical amounts of fatty acids, if used, which can include stearic acid,palmitic acid, linoleic acid or a mixture of one or more fatty acids,can comprise about 0.5 to about 3 phr. Typical amounts of zinc oxidecomprise about one to about 5 phr. Typical amounts of waxes compriseabout one to about 2 phr. Often microcrystalline waxes are used. Typicalamounts of peptizers, if used, comprise about 0.1 to about 1 phr.Typical peptizers can be, for example, pentachlorothiophenol anddibenzamidodiphenyl disulfide.

The reinforced rubber compounds can be cured in a conventional mannerwith known vulcanizing agents at about 0.1 to 10 phr. For a generaldisclosure of suitable vulcanizing agents, one can refer to Kirk-Othmer,Encyclopedia of Chemical Technology, 3rd ed., Wiley Interscience, N.Y.1982, Vol. 20, pp. 365 to 468, particularly “Vulcanization Agents andAuxiliary Materials,” pp. 390 to 402. Vulcanizing agents can be usedalone or in combination.

The vulcanization is conducted in the presence of a sulfur vulcanizingagent. Examples of suitable sulfur vulcanizing agents include“rubbermaker's” soluble sulfur; sulfur donating vulcanizing agents, suchas an amine disulfide, polymeric polysulfide or sulfur olefin adducts;and insoluble polymeric sulfur. Preferably, the sulfur vulcanizing agentis soluble sulfur or a mixture of soluble and insoluble polymericsulfur. The sulfur vulcanizing agents are used in an amount ranging fromabout 0.1 to about 10 phr, more preferably about 1.5 to about 7.5 phr,with a range of about 1.5 to about 5 phr being most preferred.

Accelerators are used to control the time and/or temperature requiredfor vulcanization and to improve properties of the vulcanizate. Thevulcanization accelerators used in the present invention are notparticularly limited. Examples include thiazol vulcanizationaccelerators, such as 2-mercaptobenzothiazol, dibenzothiazyl disulfide,N-cyclohexyl-2-benzothiazyl-sulfenamide (CBS), and the like; andguanidine vulcanization accelerators, such as diphenylguanidine (DPG)and the like. The amount of the vulcanization accelerator used is about0.1 to about 5 phr, preferably about 0.2 to about 3 phr.

Pneumatic tires having an improved tensile mechanical and dynamicviscoelastic properties, and comprising at least one component producedfrom the sulfur-vulcanized elastomeric compounds of the invention,according to the methods of the invention described above, preferablyexhibit reduced hysteresis as measured by about a 3% to about a 20%decrease in tan δ at 65° C. and/or tan δ at 50° C., and/or about a 3% toabout a 20% increase in the tensile modulus at 300% strain, compared toa tire component produced from a similar compound mixed at the hightemperature in the absence of the strong organic base.

EXAMPLES

The following examples illustrate methods of preparation of thevulcanizable elastomeric composition of the present invention. However,the examples are not intended to be limiting, as other methods forpreparing these compositions and different compounding formulations maybe determined by those skilled in the art without departing from thescope of the invention herein disclosed and claimed.

Example 1

Synthesis of a Mixture of Tin-Coupled and TEOS-Terminated SBR Polymer

This polymer was a mixture containing about 45% TEOS-terminated polymersand about 35% tin-oupled polymer. This polymer was used to prepare theinvention rubber stocks designated “Stock 1 ” and “Stock 2” andcomparison stocks “C-A” and “C-B” in Table 3.

To a one gallon reactor was charged 0.07 kg of hexane, 0.41 kg of 33.0percent by weight styrene in hexane, and 1.74 kg 22.4 percent by weightbutadiene in hexane. Then, 0.28 ml of 1.6 M 2-2′-di(tetrahydrofuryl)propane in hexane, 0.63 ml of 0.6 M potassium t-amylate in hexane, 1.42ml of 3.54 M hexamethyleneimine and 3.93 ml of 1.6 M n-butyl lithium inhexane were charged into the reactor, and the jacket temperature was setat 122° F. After 97 minutes, 2.20 ml of 0.25 M tin tetrachloride inhexane was added to the reactor. Ten minutes later, 2.53 ml of 1.12 Mtetraethoxyorthosilicate was added to the reactor. After 15 additionalminutes, the cement was discharged from the reactor, coagulated withisopropanol, treated with DBPC, and drum dried. The properties of thepolymer were: ML₁₊₄=52.4; percent chain coupling=74.5%; M_(n)=1.50×10⁵g/mol.

Example 2

Tin-Coupled SBR

A tin-coupled polymer sold commercially by Bridgestone/FirestoneCorporation was obtained. The properties of the SBR tin-coupled polymerare: ML₁₊₄=72; styrene content=20%; vinyl content=59%; and T_(g)=−33° C.This polymer was used to prepare the invention rubber stock designated“Stock 3” and comparison stock “C-C” in Table 3.

Example 3

In order to demonstrate the methods of preparation and properties of thevulcanized elastomeric compounds of the invention, six stocks of rubberswere prepared using the compounding formulation and mixing conditionsshown in Tables 1, 2 and 3. Stocks 1, 2 and 3 were compounded with DPGin the master batch to a temperature of 175° C. to 180° C. In the masterbatch stage, the DPG acts as a catalyst for the alkoxysilane-silicareaction. Comparison stocks C-A, C-B and C-C were compounded with DPG inthe final mixing stage to a temperature of 105° C. where, in combinationwith the curing agents, DPG serves its customary role as a secondaryaccelerator.

Stocks 1 and 2 and comparison stocks C-A and C-B were prepared with thetin coupled and TEOS-terminated polymers described in Example 1. Stock 1is an invention stock prepared by compounding the polymer with silica,the alkyl alkoxysilane, OTES, and DPG at high temperature in the masterbatch. C-A is a comparison stock prepared by compounding the polymerwith silica in the master batch stage at high temperature, and addingSi69 in the remill to a temperature of 155° C. Stock 2 was prepared byadding a small amount (0.2 phr) of Si69 to the polymer and silica in themaster batch, in addition to OTES, DPG, and an additional dispersingaid, SMO. C-B was prepared in the same way, except that the DPG wasadded at low temperature in the final mixing stage. For comparison withthe alkoxysilane (TEOS)-terminated stocks 1 and 2 and comparison stocksC-A and C-B, stock 3 and comparison stock C-C were prepared with atin-coupled polymer described in Example 2, and compounded with the sameingredients as stock 2 and C-B, respectively. Stocks 3 and comparisonstock C-C were not alkoxysilane-terminated polymers. The total sulfurcontent of the stocks containing less Si69 than that of C-A was adjustedto compensate for the reduction in the amount of sulfur in comparisonwith that donated by the Si69 in C-A. The final stocks were sheeted andsubsequently annealed at 171° C. for 15 minutes.

TABLE 1 Formulations of Stock Rubbers Ingredient Amount (phr)Functionalized or Tin-Coupled Solution SBR 100 (see Table 3) CarbonBlack (SAF) 35 Precipitated Silica 30 Octyl triethoxysilane (OTES)varied Diphenylguanidine (DPG) 0.5 Sorbitan Monooleate (SMO) variedBis-(3-triethoxysilylpropyl) tetrasulfide* varied Naphthenic Process Oil15 Wax 1.5 Antioxidant 0.95 Sulfur varied Accelerator, N-Cyclo-2- 1.5benzothiazolesulfenamide (CBS). Zinc Oxide 2.5 *Si69 liquid from Degussa

TABLE 2 Mixing Conditions Mixer 310 g Brabender Agitation Speed 60 rpmMaster Batch Stage Initial Temperature 100° C. 0 seconds chargingpolymers 30 seconds charging carbon black, silica, OTES, DPG (if addedin master batch), SMO (if used), Si69 (if used, 0.5 phr or less), andall pigments 5 minutes drop Drop Temperature 175° C.-180° C. RemillStage Initial Temperature 70° C. 0 seconds charging master batch stock30 seconds charging Si69 (if added) Drop Temperature 155° C. Final StageInitial Temperature 90° C. 0 seconds charging remilled stock 30 secondscharging curing agent and accelerators (including DPG if added in finalstage) Drop Temperature 105° C.

TABLE 3 Ingredients Used in Various Rubber Stocks OTES in Si69 Amountmaster DPG Amount Stock SBR Polymer and Stage of batch and Stage of SMOSulfur Number Type Addition* (phr) Addition* (phr) (phr) C-ATin-coupled + TEOS- 3 phr in RE 0 0.5 phr in final 0 1.7 terminatedStock 1 Tin-coupled + TEOS- 0 3 0.5 phr in MA 0 2.5 terminated C-BTin-coupled + TEOS- 0.2 phr in MA 1.65 0.5 phr in final 2.8 2.33terminated Stock 2 Tin-coupled + TEOS- 0.2 phr in MA 1.65 0.5 phr in MA2.8 2.33 terminated C-C Tin-coupled 0.2 phr in MA 1.65 0.5 phr in final2.8 2.33 Stock 3 Tin-coupled 0.2 phr in MA 1.65 0.5 phr in MA 2.8 2.33*MA = master batch stage RE = remill stage Final = final stage

Example 4

The green stock (i.e., the stock obtained after the final stage, priorto curing) was characterized as to Mooney viscosity and Payne effect (ΔG′) and cure characteristics. The Mooney viscosity measurement wasconducted at 130° C. using a large rotor, and was recorded as the torquewhen rotor had rotated for 4 minutes. The stocks were preheated at 130°C. for 1 minute before the rotor was started. The t₅ is the timerequired for the viscosity to increase by five Mooney units during aMooney-scorch measurement. It is used as an index to predict how fastthe compound viscosity will increase during processing (e.g., duringextrusion). The Payne effect was measured using an RPA 2000 viscometer(Alpha Technologies). The strain sweep test (ΔG′) was conducted at 50°C. at 0.1 Hz using strain sweeping from 0.25% to 1000%.

As illustrated in Table 4, the compound Mooney viscosity and the Payneeffect of invention Stock 1 are slightly higher than that of C-A whichemploys Si69 alone. However, these values are within a satisfactoryrange. Improvement in both parameters can be achieved-by the addition ofmore OTES and/or adding SMO, according to the teachings describedhereinabove. Stocks 2 and 3 and C-B and C-C all show a reduced compoundMooney viscosity and a reduced Payne effect in comparison to Stock 1 andC-A, indicating that the OTES and SMO, with the optional addition of asmall amount of Si69, produces better filler dispersion and less fillerflocculation after compounding, regardless of the presence of DPG in themaster batch. A reduced compound Mooney viscosity is advantageousbecause it provides better processability and handling, especiallyduring the extrusion process.

A Monsanto Rheometer MD2000 was used to characterize the stock curingprocess, at a frequency of 1.67 Hz and a strain of 7% at 171° C. Themeasurements t_(S2) and t₉₀ are the times taken for an increase intorque of 2% and 90%, respectively, of the total torque increase duringthe cure characterization test. These values are useful in predictingthe speed of the viscosity increase (t_(S2)) and the cure rate duringthe cure process (t₉₀). From the data illustrated in Table 4, all of theinvention Stocks 1, 2 and 3 and C-B and C-C showed a significantincrease in the scorch time, and in the t_(S2), in comparison to thecomparison Si69 stock C-A. These results reflect the decreasedavailability of polysulfide groups at the high master batch temperature,and, concomitantly, the reduced tendency for unwanted precure. Theincreased scorch time affords the stocks the advantage of a largerprocessing time window, especially during extrusion. The increase in thet_(s2) provides the stocks a longer time to flow and to better fill themold. The relatively fast curing rate of Stocks 1 and 2, compared to C-Aand the tin-coupled polymer Stock 3 and C-C, are another advantage ofthe invention stocks.

The foregoing Mooney viscosity, Payne effect and curing results supportthe notion that adding the DPG in the masterbatch does not affect therubber processability obtainable from the silica shielding agents.

TABLE 4 The green stock Mooney and Cure Characteristics t₅ Scorch t_(S2)t₉₀ Mooney @ Δ G′ (G′ @ @ @ Stock @ 130° C. 0.25%-G′ @ 1000%) 171° C.171° C. Number 130° C. (min) (kPa) (min) (min) C-A 71.4 18.29 830 1.7913.58 Stock 1 73.7 20.29 937 2.96 12.74 C-B 62.2 23.60 505 2.27 11.88Stock 2 61.5 24.82 543 2.45 12.53 C-C 50.0 27.00 737 2.51 20.82 Stock 349.9 27.60 718 2.37 19.95

Example 5

The dynamic viscoelastic mechanical properties of the cured inventionand comparison stock stocks are listed in Table 5, and were obtainedfrom strain and temperature sweep tests. The Payne effect (ΔG′) and tanδ at 7% strain (a measure of hysteresis) were obtained from the strainsweep test, which was conducted at a frequency of 3.14 radians/second at65° C. with strain sweeping from 0.25% to 14.75%. As illustrated inTable 5, each of the invention Stocks 1 and 2 show a lower Payne effectand lower hysteresis than their respective comparison stocks C-A andC-B, indicating that better microdispersion of silica is obtained bystocks in which DPG was added in the master batch at high temperature.Without being bound by theory, it is believed that the improvedmicrodispersion of silica is due to the increased binding of silica tothe alkoxysilane-terminated polymer due to the association of the polaramine group of the DPG with the polar silica surface. As expected, thetin-coupled Stock 3 containing DPG in the master batch showed improvedPayne effect and lower hysteresis compared to the comparison stock C-Cin which DPG was added in the final mixing stage. This result indicatesthat the DPG catalyzed the alkoxysilane-silica reaction of the silicaand alkyl alkoxysilane dispersing aid. However, the silicamicrodispersion in each of the tin-coupled polymer stocks was poor incomparison to the stocks using alkoxysilane-terminated polymers.

Further dynamic viscoelastic properties of the cured stock andcomparison stock compounds (i.e., the modulus at −20° C. and the tan δat 0° C. and 50° C.) were obtained as were obtained from temperaturesweep tests conducted at a frequency of 31.4 radians/second using 5%strain for the temperatures ranging from −100° C. to −10° C. and 2%strain for the temperatures ranging from −10° C. to −100° C. Asillustrated in Table 5, the modulus and tan δ properties are not aspronounced in the temperature sweep data as in the tensile mechanicalproperties listed in Table 6, below, and appear to depend more upon thetype of polymer (alkoxysilane-terminated, tin-coupled, or a mixture ofthese) than the effect of DPG in the master batch.

Example 6

The tensile properties and Lambourn abrasion indices for the six stockswere measured using the standard procedure described in ASTM-D 412 at25° C. The tensile test specimens were round rings with a diameter of0.05 inches and a thickness of 0.075 inches. A gauge length of 1.0inches was used for the tensile test. The wear resistance of the sampleswas evaluated by weighing the amount of wear using the Lambourn test.The wear index was obtained from the ratio of the weight loss of thecomparison stock to that of the tested sample. Samples with higher wearindices have better wear resistance properties. Samples used forLambourn testing were toroidal (donut-shaped) with an approximate insideand outside diameter of 0.9 inches and 1.9 inches, respectively, and athickness of approximately 0.195 inches. Test specimens were placed onan axle and run at a slip ratio of 25% against a driven abrasivesurface.

TABLE 5 The Viscoelastic Properties measured by Temperature Sweep andStrain Sweep Δ G′ @ 65° C. tan δ @ G′ @ (G′ @ 7% strain @ −20° C. tan δ@ tan δ @ Stock 0.25%-G′ @ 14.75%) 65° C. (MPa) 0° C. 50° C. Number(MPa) (S.S.) (S.S.) (T.S.) (T.S.) (T.S.) C-A 0.930 0.0968 27.2 0.29170.1347 Stock 1 0.720 0.0858 25.2 0.3046 0.1138 C-B 0.738 0.0816 19.00.2974 0.1158 Stock 2 0.587 0.0779 22.1 0.3016 0.1273 C-C 1.312 0.089659.6 0.3866 0.1293 Stock 3 1.143 0.0877 55.0 0.3718 0.1429 T.S. =Temperature Sweep data S.S. = Strain Sweep data

TABLE 6 Tensile Mechanical Properties and Lambourn Wear Index at 25° C.Elongation Lambourn Stock M 50 M 300 Strength, at Break, Toughness WearNumber (psi) (psi) Tb (psi) Eb (%) (psi) Index C-A 212 2247 3042 3744595 ND* Stock 1 211 2050 3217 410 5337 ND* C-B 190 1800 2062 310 2524100 Stock 2 198 2045 2658 380 4030 102 C-C 204 1799 2402 369 3715 100Stock 3 221 1998 2565 382 4136 101 *ND = Not Done

As illustrated by the results of the tensile and abrasion tests in Table6, each of the invention Stocks 1, 2 and 3 showed superior tensilestrength, elongation at break, and toughness, and Stocks 2 and 3 showeda comparable wear index, compared to their respective comparison stocksC-A, C-B and C-C. Moreover, the 50% elastic modulus and the 300% elasticmodulus were increased by 4% and 14%, respectively, for Stock 2,compared to C-B, showing the improved elastic modulus obtained by usingDPG in the master batch. Stock 3, having DPG in the master batch, alsoshowed an increase of 8% in the 50% elastic modulus and an increase of11% in the 300% elastic modulus compared to C-C which contained no DPGin the master batch. The 50% elastic modulus and 300% elastic modulus ofStock 1 was almost equivalent to that obtained by using Si69 in theremill where DPG was used only as a secondary accelerator in the finalstage of mixing (comparison stock C-A).

Example 7

In order to demonstrate the methods of preparation and properties of theembodiment of the invention employing a mercaptosilane silica couplingagent and alkyl alkoxysilane with the strong organic base catalyst,three stocks of rubbers were prepared using the compounding formulationand mixing conditions shown in Tables 7, 8 and 9. As illustrated inTables 8 and 9, invention stock 4 was a rubber stock compounded withsilica, octyl triethoxysilane (OTES), and3-mercaptopropyltriethoxysilane (MS) at a ratio of MS:OTES of 0.067:1,and the strong organic base, DPG, in the master batch stage to anachieved temperature of 175° C. Comparison stocks C-D and C-E werecompounded with DPG in the final mixing stage to a temperature of 105°C. where, in combination with the curing agents, DPG serves itscustomary role as a secondary accelerator. In comparison stock C-D, thepolymer was compounded with silica and a conventional amount of Si69,which was added in the remill stage to an achieved temperature of 155°C., in order to avoid premature curing which would occur at atemperature greater than 160° C. In comparison stock C-E, the polymerwas compounded in the same way as invention stock 4, except that the DPGwas absent in the master batch stage, but was added in the final mixingstage, where it served as a secondary accelerator.

The 3-mercaptopropyltriethoxysilane (MS) was used in liquid form or inthe form of Ciptane® 255 LD from PPG Industries, which is MS carried onsilica. When Ciptane® was employed, the amount of silica added to thecompound was adjusted to maintain a total silica amount of 30 phr. Thetotal sulfur content of invention stocks 4, and comparison stock C-E,was adjusted to compensate for the reduction in the amount of sulfur incomparison with that donated by the Si69 in C-D.

All of the compounded final stocks prepared as described above weresheeted and subsequently annealed at 171° C. for 15 minutes.

Example 8

The Mooney viscosity and cure characteristics of the green stock weremeasured, as described above. As illustrated in Table 10, the compoundMooney viscosity of invention Stock 4 is slightly higher than that ofC-A which employs Si69 alone, but is within a satisfactory range. Bothinvention stock 4, employing DPG in the master batch stage, andcomparison stock C-E employing DPG in the final stage showed asignificant increase in the scorch time, and in the t_(S2), incomparison to the comparison Si69 stock C-D. Moreover, the scorch timeof invention stock 4 was also significantly increased over that of C-E.The increased scorch time affords the stocks the advantage of a decreasein unwanted precuring and a larger processing time window, especiallyduring extrusion. The increase in the t_(s2) provides the stocks alonger time to flow and to better fill the mold. The very fast curingrate of stocks 4 and C-E, compared to C-E is another advantage of theuse of the mercaptosilane/alkoxysilane embodiment of the invention.

The foregoing Mooney viscosity and curing results support the notionthat adding the DPG in the masterbatch does not affect the rubberprocessability obtainable from the silica shielding agents.

TABLE 7 Formulations of Stock Rubbers Ingredient Amount (phr) SolutionSBR, tetraethoxysilane (TEOS)- 75 terminated, tin-coupled Natural Rubber25 Carbon Black (SAF) 35 Precipitated Silica 30 Silica coupling agent*varied Alkyl alkoxysilane, OTES** varied Diphenyl guanidine, DPG variedNaphthenic Process Oil 15 Wax 1.5 Antioxidant,N-(1,3-dimethylbutyl)-N′-phenyl- 0.95 p-phenylene-diamine Sulfur variedAccelerator, N-cyclohexyl-2- 1.5 benzothiazylsulfenamide (CBS) ZincOxide 2.5 *Si69 liquid (Degussa); or liquid 3-mercaptopropyltriethoxysilane or 3-mercaptopropyl triethoxysilane carried on silica(Ciptane ® 255LD from PPG Industries) **OTES = n-octyltriethoxysilane

TABLE 8 Mixing Conditions Mixer 310 g Brabender Agitation Speed 60 rpmMaster Batch Stage Initial Temperature 100° C. 0 seconds chargingpolymers 30 seconds charging carbon black, silica, mercaptosilane, OTES,DPG (if added in master batch), and all pigments 5 minutes drop DropTemperature 175° C. Remill Stage Initial Temperature 70° C. 0 secondscharging master batch stock 30 seconds charging Si69 (if added) DropTemperature 155° C. Final Stage Initial Temperature 90° C. 0 secondscharging remilled stock 30 seconds charging curing agent andaccelerators (including DPG if added only in final stage) DropTemperature 105° C.

TABLE 9 Ingredients Used in Various Rubber Stocks DPG Si69 OTES AmountAmount in MS in and Stage SBR and master master of Stock Polymer Stageof batch batch Addition* Sulfur Number Type Addition* (phr) (phr) (phr)(phr) C-D Tin 3 phr in 0 0 0.5 phr in 1.7 coupled + RE final TEOS-terminated C-E Tin- 0 3 0.2 0.5 phr in 2.5 coupled + final TEOS-terminated Stock 4 Tin- 0 3 0.2 0.5 phr in 2.5 coupled + MA TEOS-terminated *MA = master batch stage RE = remill stage Final = finalstage

TABLE 10 The green stock Mooney and Cure Characteristics t₅ Scorcht_(S2) t₉₀ Stock Mooney @ 130° C. @ 171° C. @ 171° C. Number @ 130° C.(min) (min) (min) C-D 71.2 18.48 1.79 13.58 C-E 78.4 20.48 1.86 6.13Stock 4 75.4 22.17 2.09 6.62

Example 9

As illustrated by the results of the tensile mechanical tests in Table11, the invention stock 4 and C-E showed an increased elastic modulus atboth 50% and 300% strain, compared to the C-D stock, showing theimproved elastic modulus obtained with the mercaptosilane/alkoxysilaneratio in each of these stocks. Moreover, invention stock 4 showed adesirable increase in wet traction, illustrated by the British PortableSkid Tester (BPST) result. Briefly, the portable skid tester attempts tomodel, in the laboratory, the environment in which skidding resistanceoccurs in the zone of contact between a skidding tire and a wet road.The test is described in ASTM E-303, Volume 04.03. The cured samples fortesting measured 0.98 inches to 1.00 inches in width, 2.9 inches to 3.00inches in length, and 0.25 inches to 0.27 inches in thickness. Thespecimens were tested on a concrete surface. To perform the test, thespecimen was attached to the base of the pendulum arm on the tester. Thespecimen contacts the opposing surface (the concrete) during a swing ofthe pendulum. The weighted pendulum head is free to move vertically onthe pendulum arm so that the swing amplitude is determined by thefriction of the rubber against the pavement surface. The lower theamplitude that the pendulum swings up after contacting the surface(recorded as a higher value on the scale of the tester), the higher isthe friction of the rubber against the surface.

TABLE 11 Tensile mechanical properties at 25° C. Elongation Stock M50M300 Strength, Tb at Break, Eb Toughness Number (psi) (psi) (psi) (%)(psi) BPST* C-D 212 2247 3042 374 4595 53 C-E 225 2643 3409 361 4978 53Stock 4 221 2600 2974 322 3920 57 *BPST = British Portable Skid Testerfor wet traction (see text)

Example 10

The dynamic viscoelastic mechanical properties of the cured inventionstock 4 and comparison stock C-D and C-E are listed in Table 12, andwere obtained from strain and temperature sweep tests. The Payne effect(ΔG′) and tan δ at 7% strain (a measure of hysteresis) were obtainedfrom the strain sweep test, which was conducted at a frequency of 3.14radians/second at 65° C. with strain sweeping from 0.25% to 14.75%. Asillustrated in Table 12, each of invention stock 4 and comparison stockC-E show a lower Payne effect and lower hysteresis than the Si69comparison stock C-D. Moreover, invention stock 4 has a lower Payneeffect than C-E, indicating that better microdispersion of silica isobtained by the stock in which DPG was added in the master batch at hightemperature. Without being bound by theory, it is believed that theimproved microdispersion of silica is due to the increased binding ofsilica to the polymer due to the association of the polar amine group ofthe DPG with the polar silica surface.

Further dynamic viscoelastic properties of the cured stock andcomparison stock compounds (i.e., the modulus at −20° C. and the tan δat 0° C. and 50° C.) were obtained as were obtained from temperaturesweep tests conducted at a frequency of 31.4 radians/second using 5%strain for the temperatures ranging from −100° C. to −10° C. and 2%strain for the temperatures ranging from −10° C. to −100° C. Asillustrated in Table 12, the invention stock 4 in which DPG was added inthe master batch has a desirable decreased storage modulus (G′@ −20° C.)and reduced hysteresis measured by the tan δ at 50° C., compared to theC-D and C-E stocks, showing that these dynamic viscoelastic propertiesare improved by employing the DPG catalyst in the master batch stage ofrubber compounding, using the mercaptosilane/alkyl alkoxysilaneembodiment of the invention. Reduced hysteresis of the invention stock4, compared with both C-D and C-E is further illustrated in FIG. 1,which is a temperature sweep curve.

TABLE 12 The viscoelastic properties measured by temperature and strainsweeps tan δ @ Δ G′ @ 65° C. 7% G′ @ (G′ @ strain @ −20° C. tan δ @ tanδ @ Stock 0.25%-G′ @ 14.75%) 65° C. (MPa) 0° C. 50° C. Number (MPa)(S.S.) (S.S.) (T.S.) (T.S.) (T.S.) C-D 0.93 0.0968 27.2 0.2917 0.1347C-E 0.61 0.0669 26.8 0.3197 0.1188 Stock 4 0.57 0.0702 20.4 0.29400.0943 T.S. = Temperature Sweep data S.S. = Strain Sweep data

In conclusion, the invention stocks produced by mixing DPG in the masterbatch at high temperatures showed surprisingly superior physicalproperties in comparison with stocks produced by adding DPG in the finalmixing stage. Moreover, the invention stocks showed comparable orimproved physical properties compared with stocks preparedconventionally with Si69 in the remill, where the DPG was added only inthe final stage of mixing. In particular, the improvements in tensilestrength elongation at break and toughness indicated that bondingbetween TEOS polymers and silica fillers can be enhanced through thesilanization reaction that is catalyzed by the presence of DPG in themaster batch. This bonding contributes to a higher degree of rubberreinforcement and results in better mechanical properties.

While the invention has been described herein with reference to thepreferred embodiments, it is to be understood that it is not intended tolimit the invention to the specific forms disclosed. On the contrary, itis intended that the invention cover all modifications and alternativeforms falling within the scope of the appended claims.

1. A sulfur-vulcanizable elastomeric compound, comprising: an elastomer;a reinforcing filler comprising silica or a mixture thereof with carbonblack; an alkyl alkoxysilane; a mercaptosilane silica coupling agent,wherein the weight ratio of the mercaptosilane to the alkyl alkoxysilaneis a maximum of 0.14:1; a catalytic amount of a strong organic base; anda cure agent comprising an effective amount of sulfur to achieve asatisfactory cure, wherein the elastomer, the silica, the alkylalkoxysilane, the mercaptosilane and the strong organic base are mixedtogether, in the absence of the cure agent, at a temperature of about130° C. to about 200° C.
 2. The compound of claim 1, wherein thecompound exhibits an improved tensile mechanical or dynamic viscoelasticproperty, selected from the group of properties consisting of about a 1%to about a 10% decrease in Mooney viscosity after compounding, reducedhysteresis as measured by about a 3% to about a 20% decrease in tan σ at50° C., about a 3% to about a 20% decrease in filler flocculation aftercompounding, and combinations thereof, compared to a similar compoundmixed at the temperature in the absence of the strong organic base. 3.The compound of claim 1, wherein the strong organic base is selectedfrom the group consisting of a guanidine, a hindered amine base, andmixtures thereof.
 4. The compound of claim 3, wherein the guanidine isselected from the group consisting of triphenylguanidine,diphenylguanidine, di-o-tolylguanidine, and mixtures thereof.
 5. Thecompound of claim 4, wherein the guanidine is diphenylguanidine.
 6. Thecompound of claim 3, wherein the hindered amine base is selected fromthe group consisting of 1,8-diazabicyclo[5.4.0]undec-7-ene;1,5-diazabicyclo[4.3.0]non-5-ene; and mixtures thereof.
 7. The compoundof claim 1, wherein the catalytic amount of the strong organic base isabout 0.1% to about 10% by weight, based on the weight of the silica. 8.The compound of claim 7, wherein the catalytic amount of the strongorganic base about 0.1% to about 5% by weight, based on the weight ofthe silica.
 9. The compound of claim 1, wherein the weight ratio of themercaptosilane to the alkyl alkoxysilane is about 0.001:1 to about0.10:1.
 10. The compound of claim 1, wherein the weight ratio of themercaptosilane to the alkyl alkoxysilane is about 0.01:1 to about0.10:1.
 11. The compound of claim 1, wherein the mercaptosilane ispresent in an amount of about 0.0001% to about 3% by weight, based onthe weight of the silica.
 12. The compound of claim 1, wherein themercaptosilane is present in an amount of about 0.001% to about 1.5% byweight, based on the weight of the silica.
 13. The compound of claim 1,wherein the mercaptosilane is present in an amount of about 0.01% toabout 1% by weight, based on the weight of the silica.
 14. The compoundof claim 1, wherein the alkyl alkoxysilane is an alkyl trialkoxysilane.15. The compound of claim 1, further comprising a silica dispersing aidselected from the group consisting of a fatty acid ester of ahydrogenated or non-hydrogenated C₅ or C₆ sugar, a polyoxyethylenederivative of a fatty acid ester of a hydrogenated or non-hydrogenatedC₅ or C₆ sugar, an ester of a polyol, and mixtures thereof.
 16. Thecompound of claim 15, wherein the fatty acid ester is selected from thegroup consisting of sorbitan monooleate, sorbitan dioleate, sorbitantrioleate, sorbitan sesquioleate, sorbitan laurate, sorbitan palmitate,sorbitan stearate, and mixtures thereof.
 17. The compound of claim 1,wherein the elastomer is selected from the group consisting ofhomopolymers of conjugated diene monomers, and copolymers andterpolymers of the conjugated diene monomers with monovinyl aromaticmonomers and trienes.
 18. A sulfur-vulcanizable elastomeric compoundhaving improved tensile mechanical properties and dynamic viscoelasticproperties, comprising: an elastomer; a reinforcing filler comprisingsilica or a mixture thereof with carbon black; a silica coupling agentselected from the group consisting of about 0.01% to about 1% by weightof a bis(trialkoxysilylorgano) tetrasulfide silica coupling agent, basedon the weight of the silica, about 0.1% to about 20% by weight of abis(trialkoxysilylorgano) disulfide silica coupling agent, based on theweight of the silica, and mixtures thereof; a silica dispersing aid; acatalytic amount of a strong organic base; and a cure agent comprisingan effective amount of sulfur to achieve a satisfactory cure, whereinthe elastomer, the silica, the silica coupling agent, the silicadispersing aid, and the strong organic base are mixed together, in theabsence of the cure agent, at a temperature of 165° C. to about 200° C.19. The compound of claim 18, wherein the compound exhibits an improvedtensile mechanical or dynamic viscoelastic property compared to asimilar compound mixed at the temperature in the absence of the strongorganic base, the property selected from the group consisting of about a10% to about a 40% decrease in filler flocculation after compounding, asmeasured by ΔG′; reduced hysteresis as measured by about a 3% to about a20% decrease in tangent σ at 650° C.; about a 3% to about a 20% increasein the tensile modulus at 300% strain, and combinations thereof.
 20. Thecompound of claim 18, wherein the strong organic base is selected fromthe group consisting of a guanidine, a hindered amine base, and mixturesthereof.
 21. The compound of claim 20, wherein the guanidine is selectedfrom the group consisting of triphenylguanidine, diphenylguanidine,di-o-tolylguanidine, and mixtures thereof.
 22. The compound of claim 21,wherein the guanidine is diphenylguanidine.
 23. The compound of claim20, wherein the hindered amine base is selected from the groupconsisting of 1,8-diazabicyclo[5.4.0]undec-7-ene;1,5-diazabicyclo[4.3.0]non-5-ene; and mixtures thereof.
 24. The compoundof claim 18, wherein the catalytic amount of the strong organic base isabout 0.1% to about 10% by weight, based on the weight of the silica.25. The compound of claim 24, wherein the catalytic amount of the strongorganic base about 0.1% to about 5% by weight, based on the weight ofthe silica.
 26. The compound of claim 18, wherein the silica dispersingaid is selected from the group consisting of an alkyl alkoxysilane, afatty acid ester of a hydrogenated or non-hydrogenated C₅ or C₆ sugar, apolyoxyethylene derivative of a fatty acid ester of a hydrogenated ornon-hydrogenated C₅ or C₆ sugar, an ester of a polyol, and mixturesthereof.
 27. The compound of claim 26, wherein the alkyl alkoxysilane isa triethoxysilane.
 28. The compound of claim 26, wherein the fatty acidester is selected from the group consisting of sorbitan monooleate,sorbitan dioleate, sorbitan trioleate, sorbitan sesquioleate, sorbitanlaurate, sorbitan palmitate, sorbitan stearate, and mixtures thereof.29. The compound of claim 18, wherein the elastomer is selected from thegroup consisting of homopolymers of conjugated diene monomers, andcopolymers and terpolymers of the conjugated diene monomers withmonovinyl aromatic monomers and trienes.
 30. The compound of claim 1,wherein the elastomer comprises an alkoxysilane terminal group.
 31. Thecompound of claim 18, wherein the elastomer comprises an alkoxysilaneterminal group.